Photonic crystal organic light emitting device having high extraction efficiency

ABSTRACT

Disclosed is a photonic crystal organic light emitting device. The device comprises a transparent substrate having a concavo-convex structure in an upper surface thereof, a transparent electrode layer formed on the transparent substrate, a hole conduction layer comprised of an organic EL material and formed on the transparent electrode layer, an electron conduction layer comprised of an organic EL material and formed on the hole conduction layer, and a cathode layer formed on the electron conduction layer. Here, a photonic crystal period due to the concavo-convex structure formed in the upper surface of the transparent substrate corresponds to a wavelength level of light generated at an interface between the electron conduction layer and the hole conduction layer. Therefore, a light extraction efficiency and a viewing angle are increased without degeneration of the image quality. Further, since the photonic crystal has an effective refractive value ranged between that of the transparent substrate and that of the transparent electrode layer, it has the same effect as in nonreflective coating, thereby simultaneously increasing the transmittance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an organic light emittingdevice, and more particularly, to an organic light emitting device inwhich a photonic crystal concavo-convex structure is formed in atransparent substrate and an transparent electrode layer to increase alight extraction efficiency.

[0003] 2. Description of the Related Art

[0004] Recently, display devices based on organic light emittingmaterials become the center of attraction due to flatness, highdefinition, portability, low power consumption and so forth.

[0005]FIG. 1 is a schematic view of a conventional organic lightemitting device. Referring to FIG. 1, the conventional organic lightemitting device has a structure in which a transparent electrode layer20, a hole conduction layer 30, an electron conduction layer 40 and acathode layer 50 are sequentially stacked on a transparent substrate 10.

[0006] Herein, a glass substrate is typically used as the transparentsubstrate 10. An ITO (Indium-Tin-Oxide) layer is mainly used as thetransparent electrode layer 20. In addition, an Mg—Al alloy layer may beused as the cathode electrode layer 50. The hole conduction layer 30 andthe electron conduction layer 40 are comprised of an organicEL(electroluminescent) material. Typically, the hole conduction layer 30is comprised of N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-4,4′-diamine(hereinafter, “TPD”) or polyethylenedeoxythiophene (PEDOT), and theelectron conduction layer 30 is widely comprised of tris(8-hydroxyquinolino) aluminum (hereinafter, “Alq3”). The typicalmaterial of each layer has an absolute refractive index, i.e.,n(glass)=1.46, n(ITO)=1.8, n(TPD)=1.76 and n(Alq3)=1.7.

[0007] As shown in FIG. 1, when a negative voltage is applied to thecathode electrode layer 50 and a positive voltage is applied to thetransparent electrode layer 20, the combination of a hole and anelectron is occurred in a junction portion (35: hereinafter, “activearea”) of the hole conduction layer 30 and the electron conduction layer40. Thus, light is spontaneously radiated.

[0008] The light generated at the active area 35 is radiated through inturn an interface of the hole conduction layer 30 and the transparentelectrode layer 20 and an interface of the transparent electrode layer20 and the transparent substrate 10 to air. Since the absoluterefractive index (n(ITO)=1.8) of the transparent electrode layer 20 islarger than that (n(Alq3)=1.7) of the electron conduction layer 40, atthe interface of the electron conduction layer 40 and the transparentelectrode layer 20, most of the light is refracted toward thetransparent electrode layer 20 and then transmitted through thetransparent electrode layer 20.

[0009] However, since the absolute refractive index (n(glass)=1.8) ofthe transparent electrode layer 20 is larger than the refractive indexof substrate layer 10 (n(glass)=1.46), the light, at an angle largerthan a critical angle, is totally reflected so as to be not transmittedto the glass. Further, since the absolute refractive index of thetransparent substrate 10 is 1.46 and the absolute refractive index ofthe air is 1, the same phenomenon occurrs at the interface of thetransparent substrate 10 and the air.

[0010] In the drawing, a reference symbol θcc designates a criticalangle between the transparent electrode layer 20 and the transparentsubstrate 10, and a reference symbol θc is a critical angle between thetransparent substrate 10 and the air, and θo is an incident angle of thelight which is incident to the transparent substrate 10 to be convertedinto the angle of θc.

[0011] Assuming that the distribution of light generated from a specificradiation point of the active area 35 is spacially isotropic and thelight is not reabsorbed, the amount of light, that is totally reflectedfrom the transparent substrate, can be calculated by a followingequation: ∫_(θ  o)^(θ  cc)T_(glass)(θ)sin   θ  θ.

[0012] It is about 31.5%, wherein T_(glass) (θ) is a transmittance ofthe transparent substrate 10. And, in the same condition, the amount oflight, that is totally reflected from the transparent electrode layer20, can be calculated by a following equation:∫_(θ  cc)⁹⁰T_(ITO)(θ)sin   θ  θ.

[0013] It is about 51%, wherein T_(ITO)(θ) is a transmittance of thetransparent electrode layer 20. To summarize, the total-reflected lightamount is about 80%.

[0014] Therefore, in conventional organic light emitting devices, thelight extraction efficiency is only about 20%. There is a big room forimprovement. Because of the low light extraction efficiency, the powerdissipation should be large, and the life time of the arrayed lightemitting device is reduced. Therefore, one need to make the lightextraction efficiency as large as possible.

[0015] In order to increase the light extraction efficiency, severalschemes have been proposed. For example, a cone-shaped array is formedon a glass substrate, such that the light, entering at larger anglesthan the critical angle, can be transmitted to an outside (cf. Highexternal quantum efficiency organic light emitting device, G. Gu, D. Z.Garbuzov, P. E. Burrows, S. Venkatesh, S. R. Forrest, Optics Letters,22, 396, 1997). Or a laminated lens array is formed on a glass substrateto reduce the incident angle, thereby increasing the light extractionefficiency (cf. Improvement of output coupling efficiency of organiclight emitting diodes by backside substrate modification, C. F. Madigan,M. H. Lu, J. C. Sturm, Applied Physics Letters, 27, 1650, 2000). Inthese methods, however, there are some problems related to fabricatingmethods and the image quality is poor.

SUMMARY OF THE INVENTION

[0016] Therefore, the object of the present invention is to provide anorganic light emitting device in which a photonic crystal concavo-convexstructure is formed in the transparent substrate 10 and the transparentelectrode layer 20, thereby increasing the light extraction efficiency.

[0017] To achieve the aforementioned object of the present invention,the photonic crystal organic light emitting device includes atransparent substrate having a concavo-convex structure in an uppersurface thereof, a transparent electrode layer formed on the transparentsubstrate, a hole conduction layer comprised of an organic EL materialand formed on the transparent electrode layer, an electron conductionlayer comprised of an organic EL material and formed on the holeconduction layer, and a cathode layer formed on the electron conductionlayer.

[0018] Preferably, the lattice constant of the concavo-convex structureformed in the upper surface of the transparent substrate ranges from$\frac{1}{3}$

[0019] λ to 2λ, where λ is the wavelength of light in the active area.And transparent electrode layer has a thickness of 30-200 nm. Further,the depth of a concave potion is formed as deep as possible within theextent that the electrical properties of the transparent electrode layerare acceptable.

[0020] The photonic crystal of the concavo-convex structure formed inthe upper surface of the transparent substrate can be periodically andrepeatedly arrayed in a square lattice type, a triangular lattice typeor a honeycomb lattice type. The concavo-convex structure can be formedby etching the upper surface of the transparent substrate. However, itis clear that other methods such as a wet etching or a micro-imprintingalso can be used.

[0021] If one wants to obtain the constant diffraction angleirrespective of the color of light generated at the interface betweenthe electron conduction layer and the hole conduction layer, it isbetter to have a constant value λ/Δ, wherein Δ is a period of thephotonic period due to the concavo-convex structure formed in thetransparent substrate, and λ is a wavelength of light incident on thephotonic crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above objects and other advantages of the present inventionwill become more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

[0023]FIG. 1 is a schematic view of a conventional organic lightemitting device;

[0024]FIGS. 2 through 6 are views showing the organic light emittingdevices according to the present invention; and

[0025]FIG. 7 illustrates a simulation example for the square latticepattern which has the total 300 nm EL layer, 200 nm ITO layer, 70 nmpattern depth and wavelength 520 nm, lattice constant 400 nm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Now, preferred embodiments of the present invention will bedescribed in detail with reference to the annexed drawings.

[0027]FIGS. 2 through 6 are views showing organic light emitting devicesaccording to the present invention. Herein, the same reference numeralsas those in FIG. 1 designate components performing the same functions.

[0028]FIG. 2 is a schematic view of an organic light emitting deviceaccording to the present invention. Reference to FIG. 2, the organiclight emitting device has a structure in which a transparent electrodelayer 20, a hole conduction layer 30, an electron conduction layer 40and a cathode layer 50 are sequentially stacked on a transparentsubstrate 10, like in the conventional light emitting device

[0029] However, there is a remarkable difference from the conventionallight emitting device in that photonic crystal is formed in an uppersurface of the transparent substrate 10, which is contacted with thetransparent electrode layer 20, by a concavo-convex structure. Thephotonic crystal structure is formed in the upper surface of thetransparent substrate 10 to a desired depth. Since the etchingtechnology has been developed for a long time for semiconductor devicefabricating process, the etching process can be precisely performed. Ofcourse, it is also possible to form a protrusion on the upper surface ofthe transparent substrate 10.

[0030] After the concavo-convex structure is formed in the upper surfaceof the transparent substrate 10, the concavo-convex structure having thesame shape is also formed in an upper surface of the transparentelectrode layer 20. Of course, if the transparent electrode layer 20 isformed to be thick the concavo-convex structure formed in thetransparent electrode layer 20 may be flattened. Even in this case, theeffect of the present invention is still effective.

[0031]FIG. 3 shows various forms of the photonic crystal structure to beformed in the upper surface of the transparent substrate 10. As shown inFIGS. 3a to 3 d, the photonic crystal may be formed in a square latticetype, a triangular lattice type, a honeycomb lattice type or a randomlattice pattern.

[0032] Preferably, a period of the photonic crystal formed in thetransparent substrate 10 is a third times to two times as large as awavelength of light in the transparent electrode 20, and a depth of aconcave portion is 10-200 nm. It is preferable that the depth of theconcave portion is formed as deep as possible within an extent that theelectrical properties of the transparent electrode layer are acceptable.

[0033]FIG. 4 shows a one-dimensional grating-shaped model to explain aneffect of the present invention. In FIG. 4, a reference symbol k_(in)designates a wave vector in a plane direction, a reference symbol N isan effective refractive index at the photonic crystal (herein,n(glass)<N<n(ITO), β_(in) is 2π/λ (herein, λ is a wavelength of incidentlight), and K is 2π/Δ (herein, Δ is a period of the photonic crystal).

[0034] The light designated by the reference symbol a is the light whichis locked in the concave portion not to be radiated into the air. Thelight designated by the reference symbol b is leaky wave radiated to theoutside. The leakage light is radiated in θ direction to satisfy thefollowing equation: n(glass or ITO)·sin θ=N·k_(in)+q·K, wherein q is anintegral number.

[0035] Since the direction of the leakage wave can be controlledaccording to the period of the photonic crystal, the period may bedifferent according to a function of the light emitting device. In orderto maximize the efficiency of light and an object of the displayingdevice, it is preferable to have the major direction of the leaky wave.

[0036] Typically, since an ITO film as the transparent electrode layer20 is formed in thickness 30-200 nm, it is impossible to distinguish aposition of an image formed by the light transmitted at an angle whichis less than the critical angle from a position of an image formed bythe leaky wave with the naked eye. Therefore, the image quality is notdegraded by the leakage wave in the displaying device.

[0037] As shown in FIG. 3, if the photonic crystal is formed as atwo-dimensional structure, the leaky wave is also diffracted andscattered in various directions. Thus, the light extraction efficiencyshould increase.

[0038]FIG. 5 shows an embodiment in which the light emitting device isapplied to a color displaying device. In the color display device, ifthe photonic crystal is formed in the two-dimensional periodic arraymethod, a diffraction angle toward the outside varies depending on thewavelength of the light generated from the active area 35. Therefore, inorder to obtain the constant diffraction angle regardless of thewavelength of the light generated from the active area 35, the photoniccrystal period Δ in each color pixel needs to satisfy the followingequation: sin θ=λ_(red)/Δ=λ_(green)/Δ=λ_(blue)/Δ.

[0039] The photonic crystal lattice can also be randomly arrayed with adesired mean period. as shown in FIG. 3d, the color deflectionphenomenon in which the light having a specific wavelength is deflectedto a specific direction can be removed.

[0040]FIGS. 6a and 6 b show various concavo-convex structures formed inthe upper surface of the transparent substrate 10. As shown in drawings,the concavo-convex structure may have a curved shape (FIG. 6a) or asaw-toothed shape (FIG. 6b) instead of the vertical groove shape. Thelight extraction efficiency can be increased by the properly modulatedstructure.

[0041]FIG. 7 illustrates a simulation example for the square latticepattern which has the total 300 nm EL layer, 200 nm ITO layer, 70 nmpattern depth and wavelength 520 nm, lattice constant 400 nm. In theFIG. 7, the solid line and dotted line represent unpatterned andpatterned cases, respectively. It demonstrates that the extractionefficiency of the patterned case can be over two times larger than thatof the conventional unpatterned case.

[0042] According to the present invention, as described above, thephotonic crystal concavo-convex structure is formed in the transparentsubstrate to generate the leaky wave, thereby increasing the lightextraction efficiency and the viewing angle without degeneration of theimage quality. Further, since the effective refractive angle of thephotonic crystal is ranged between the effective refractive angle of thetransparent substrate 10 and the effective refractive angle of thetransparent electrode layer 20, the photonic crystal provides the sameeffect as in nonreflective coating, thereby simultaneously increasingthe transmittance.

[0043] While the present invention has been described in detail, itshould be understood that various changes, substitutions and alterationscould be made hereto without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A organic light emitting device, comprising: atransparent substrate having a concavo-convex structure in an uppersurface thereof; a transparent electrode layer formed on the transparentsubstrate; a hole conduction layer comprised of an organic EL materialand formed on the transparent electrode layer; an electron conductionlayer comprised of an organic EL material and formed on the holeconduction layer; and a cathode layer formed on the electron conductionlayer.
 2. The device of claim 1, wherein an absolute refractive index ofthe transparent electrode layer is larger than an absolute refractiveindex of the transparent substrate, and an absolute refractive index ofthe transparent substrate is larger than an absolute refractive index ofair.
 3. The device of claim 2, wherein the transparent electrode layeris an ITO film.
 4. The device of claim 3, wherein the transparentsubstrate is comprised of glass.
 5. The device of claim 1, wherein thetransparent electrode layer has a thickness of 30-200 nm.
 6. The deviceof claim 1, wherein an average spacing due to the concavo-convexstructure formed in the upper surface of the transparent substrate isone-third times to two times as large as a wavelength of light generatedat an interface between the electron conduction layer and the holeconduction layer.
 7. The device of claim 1, wherein a concave portion ofthe concavo-convex structure formed in the upper surface of thetransparent substrate has a depth of 10-200 nm.
 8. The device of claim1, wherein the area ratio of concave and convex of the concavo-convexstructure formed in the upper surface of the transparent substrate has20% to 80%.
 9. The device of claim 1, wherein the convo-convex shape canbe deformed to increase the extraction efficiency, for instance to therectangular or triangular shape and so on.
 10. The device of claim 1,wherein an average spacing due to the concavo-convex structure formed inthe upper surface of the transparent substrate is periodically andrepeatedly arrayed in a square lattice type.
 11. The device of claim 1,wherein an average spacing due to the concavo-convex structure formed inthe upper surface of the transparent substrate is periodically andrepeatedly arrayed in a triangular lattice type.
 12. The device of claim1, wherein an average spacing due to the concavo-convex structure formedin the upper surface of the transparent substrate is periodically andrepeatedly arrayed in a honeycomb lattice type.
 13. The device of claim10, wherein the concavo-convex locations are deviated randomly up to ½lattice constant from nature position.
 14. The device of claim 10,wherein the concavo-convex locations are deviated randomly up to ½lattice constant from nature position.
 15. The device of claim 10,wherein the concavo-convex locations are deviated randomly up to ½lattice constant from nature position.
 16. The device of claim 1,wherein the concavo-convex structure formed in the upper surface of thetransparent substrate is formed by etching the upper surface of thetransparent substrate.
 17. The device of claim 1, wherein thetransparent electrode layer has the same concavo-convex structure asthat formed in the upper surface of the transparent substrate.
 18. Thedevice of claim 1, wherein λ/Δ has a constant value, wherein Δ is aperiod of the photonic period due to the concavo-convex structure formedin the transparent substrate, and λ is a wavelength of light incident tothe photonic crystal.
 19. The device of claim 1, wherein an absoluterefractive index of the hole conduction layer is less than an absoluterefractive index of the transparent electrode layer.
 20. The device ofclaim 1, wherein the concavo-convex structure formed in the uppersurface of the transparent substrate has a curved shape or a saw-toothedshape.